In order to reduce the overall error in a Rockwell hardness measurement, it is important that the different sources of error are identified and the significant error sources be reduced if possible. This can only be accomplished by assessing the separate parameters of the Rockwell test and verifying whether each is within acceptable limits. The test method standards specify two categories of methods that can be used to assess many of the aspects of the test. They are: direct verification and indirect verification of the hardness machine. Direct verification is a process for verifying that critical components of the hardness machine are within allowable tolerances by directly measuring such parameters as the test forces, depth measuring system, and machine hysteresis. Indirect verification is a process for verifying the measurement performance of the hardness machine by performing Rockwell hardness tests using standardized reference blocks and indenters.
Good Practice Recommendations
• Although ASTM and ISO test method standards presently do not require periodic direct verification of Rockwell machines, it is recommended that both direct and indirect verifications be performed periodically based on the usage and condition of the individual machine.
• When a testing machine fails to pass indirect verification of one or more Rockwell scales, direct verification should be used as a tool to determine the source of the problem rather than making blind adjustments of a machine component or electronic offsets to correct errors.
• When a testing machine fails to pass direct verification of one or more of its components, and cannot be brought within tolerances, it should be repaired or replaced.